This invention relates generally to a signal generator based on the Direct Frequency Synthesis Method, and more particularly to an improved signal generator in which phase control at the time of frequency switching is made easier, to allow the frequency to be switched in a continuous phase.
The Direct Frequency Synthesis Method has been known as one of the basic technologies for frequency synthesis as described in Chapter III of, "Frequency Synthesis: Techniques and Application," edited by Jerzy Gorski-Popiel, staff member of MIT Lincoln Laboratory, and published by The Institute of Electrical and Electronics Engineers, Inc., New York, 1975. The Direct Frequency Synthesis Method has many advantages. For example, when the upper limit frequency is high, signal purity is also high. Also, the frequency can be switched at a high speed. But, this method has also the great disadvantage of having difficult phase control at the time of frequency switching.
FIG. 1 shows a conventional signal generator with a very simple construction which is based on the above method. As shown, a plurality of second reference frequency generators A.sub.1 -A.sub.K receives a signal of a first reference frequency f.sub.1 and respectively produces signals having a second different reference frequency f.sub.21 -f.sub.2K each of which is a multiple of the first reference frequency f.sub.1. A select switch 1 operates in response to an externally applied command requesting a change of the frequency setting data. In operation, the select switch 1 selectively switches the second reference frequencies f.sub.21 -f.sub.2K from one to another according to the command applied. The selected and switched second reference frequency signal is then applied to a mixer 2 where it is mixed with a signal of a third reference frequency f.sub.3 as a carrier signal. The output signal of a frequency, as a sum or a difference of the component between those frequencies, is filtered out by a band-pass filter 3.
In the signal generator thus constructed, however, the phases of the second reference frequencies K, f.sub.21 -f.sub.2K, are not aligned in phase.
The frequency setting data is changed absolutely independently from the operation of the reference frequency generators. In a digital circuit for transmitting the change of the frequency setting data to the select switch 1, how to speed up the switching operation is the most urgent and serious problem to be solved. Therefore, it is common practice that the digital circuit be designed so as to have a minumum delay time. It is for this reason that the switching timing of the select switch 1 is absolutely independent of the signals generated from the reference frequency generators and is also independent of each of the phases of the second reference frequency signals of K.
With such a design, at the time of frequency switching, there is no continuity between the final phase of a reference frequency before frequency switching and the first phase of another reference frequency after frequency switching. The duration of the phase confusion depends on the phase discontinuity between the reference frequencies causes by frequency switching. The phase confusion is further elongated by the passage of a signal through the band-limiting devices such as the band-pass filter 3 and the succeeding low-pass filter (not shown). As a result, the signal from the signal generator, which has the reference frequency selected anew by the select switch 1, takes a long time to settle down at the final phase after the start of the switching of the select switch 1.
The long confusion of the phase of the output signal of the switched reference frequency is equivalent to a situation when the signal is deeply phase-modulated. Accordingly, a tremendous number of spurious components appear in the form of side bands of the carrier f.sub.3 in the output signal until the phase settles down.
As described above, in the conventional signal generator based on the Direct Frequency Synthesis Method, the phase is discontinuous between the reference frequencies at the time of the reference frequency switching, and a great number of spurious components appear for a long period of time. Thus, when the conventional signal generator is applied in a device requiring frequent switching of the frequency, the spurious components can create a serious problem.
The signal generator has been used as a signal source or a local signal generator in a measuring system having a frequency selecting function such as a spectrum analyzer or a network analyzer. In such an application, the spurious components in question provide a major source of measuring errors. In an extreme case, the measurement can not be temporarily conducted. This makes the measuring device unable to perform high speed measuring. In some systems, for example, a satellite communication system, the spurious components make communication impossible. Further, when the signal generator is applied to an exciting oscillator of an elementary particle accelerator, the phase discontinuity occuring in the fine adjustment of the frequency can impair the accelerator or can stop the acceleration of the elementary particles.
In Chapter II, pp. 39 to 44 of the above article, there is described a direct digital synthesizer which could solve the above problem and realize phase continuity of an output signal before and after the switching of a frequency.
In the direct digital synthesizer, phase data is stored in a ROM. The stored phase data is read out by a given clock signal, and is converted into an analog voltage by a D/A converter, thereby obtaining a sinusoidal wave signal of a predetermined frequency. The output frequency is variable by changing the period for reading out the phase data by the clock signal.
In this direct digital synthesizer, the digital processing speed and the number of bits depend on the IC fabrication technique. The present stage of the IC fabrication technology can provide the upper-limit frequency which is much lower than that obtained by the above-mentioned signal generator designed on the basis of the Direct Frequency Synthesis Method. The purity of the signal is also poor.
For the above reasons, there has been a strong demand for the advent of a signal generator based on the Direct Frequency Synthesis Method having a frequency capable of high-speed switching, a signal having a high degree of purity, and a high upper frequency, and which is so improved that phase continuity is secured at the time of frequency switching.